Purpose
=======
Move the code which updates the mail message from the message model to mail
thread. Most other thread methods (like `_message_update_content_after_hook`)
are defined at record model level. It makes sense to delegate the update to
documents and not to the message. Message is a low-level technical object
that should not really hold business code.
While modifying this code, an update is done in the update content. We now
also allow to update the body without removing all attachments.
Finally tests are added as this feature was added without really testing
model code.
Task-2207626 (Rating: Delay rating notification to ease feedback)
Part-of: odoo/odoo#95623
Co-authored-by: Thibault Delavallée <tde@odoo.com>
